Neanderthals outcompeted by humans?

Wouldn't you love it if the Neanderthals hadn't gone extinct, but were still living with us today? I'd give my right arm to see that (but then again, I'd give my right arm to be ambidextrous). It is still hotly debated how they went extinct, but a paper in PLoS ONE [1] concludes that Homo neanderthalensis were outcompeted by humans.

The authors mention two competing hypotheses, namely

the Neanderthals were unable to adapt to the changing environment, and

I have, however, seen two or three other hypotheses before that deserves to be mentioned:

Neanderthals and AMH interbred and the distinction between them disappeared,

they perished because they didn't publish weren't able to evolve resistance to some pathogen or other, and of course

that they were wiped out by AMH in bloody feuds that so seem to define the winners to this day.

The fourth of these could be covered by the first about the changing environment (except that in this study changing environment refers strictly to climate changes), and the third hypothesis about interbreeding has been ruled out by the sequencing of a complete Neanderthal genome [2], from which they inferred that the Neanderthal mitochondrial DNA (mtDNA) falls outside of the variation among humans. Competition does not usually involve direct contact, but it might have been a mix of both competition and fighting, perhaps even with the added benefit of a nice proteinaceous meal?

The idea behind the competition hypothesis is that for longer periods of time, only one species can exist in a single niche. A niche is a nice way to describe something which is very difficult to define, but it largely means "way of life," and is comprised of all the resources that the species utilize in order to survive. Figuring out exactly what those resources are for a given species is not easy at all, because it concerns not only the foods that are eaten, but, in the words of David Tilman

any substance or factor which can lead to increased growth rates as its availability in the environment is increased, and which is consumed by an organism. [3]

Substances that can be a resource include (but may not be limited to) chemicals, other living organisms, and space (but note that if you read this paper you'll find they use 'niche' as meaning 'geographical range').

If two species, or populations, are sharing a niche (or overlap to a large extent), then they will compete for the resources, and one will necessarily drive the other to extinction given enough time (though not a lot is needed on evolutionary time-scales). The selective pressure to utilize different resources can even drive two populations of the same species away from each other, resulting in a speciation event. The two species can then coexist exactly because they no longer utilize the same resources, and thus do not occupy the same niche anymore. This happens to be one of the research projects that I am working on at the moment, so enough about that.

The study looked at the three climatic periods to evaluate whether climate change can have been the cause of the Neanderthal extinction. The three periods are

"Pre-H4": 43.3–40.2 kyr with mild weather,

"H4": 40.2–38.6 kyr with cold weather, and

"GI8": 38.6–36.5 kyr with mild weather again.

That means that the relevant time-period is from 43,300 years ago to 36,500 years ago.

The computer simulation they applied (GARP, download) used archeological data from Neanderthal and AMH archeological sites to pinpoint their geographical ranges, as well as data about the landscape and climatic dimensions potentially relevant to shaping the distribution of the species. The results are that the geographical range of the Neanderthals was much smaller in GI8 than what would be expected based on the climatic data. Thus, they conclude, the vanishing distribution of Neanderthals was not due to problems coping with the changing environment, and must then have been due to competitive exclusion by AMH. The last words in the paper's discussion are these:

The AMH expansion and Neanderthal contraction of niche characteristics were concurrent, and we suspect causally related. It follows that there was certainly contact between the two populations, which may have permitted both cultural and genetic exchanges. Our findings clearly contradict the idea that Neanderthal demise was mostly or uniquely due to climate change [51] and looks towards AMH expansion as the principal factor. Hence, we contend that AMH expansion resulted in competition with which the Neanderthal adaptive system was unable to cope.

So what they have done is to rule out one hypothesis (hyp. #1), and, without further proof, sort of succumb to the only other they list (hyp. #2), but which they did not investigate at all. As I mentioned earlier hyp. #3 has been ruled out, and hyp. #5 is perhaps part of hyp. #2 (driven to extinction by competition for resources and/or by direct hostile contact with humans). But since the Neanderthal population was very small towards the end, I don't think we can totally rule out that some disease had something to do with their demise. Additionally, it would be really nice with some direct evidence in support of the competitive exclusion hypothesis, which for example could take the form of data indicating that the resources utilized by the two Homo species were identical. Perhaps that already exists, in which case it would have been nice if it had been mentioned in this paper.

4 comments:

Um, hold up, the entire human population is rather homogenous, a fact which indicates a bottleneck population in our short-term evolutionary past (IRCC, circa 15kya, but I'm not sure where I pick this number from, it's been a while since I read popular science books about paleoanthropology) - What I mean to say by all of this is that reviewing the variance within MtDNA in humans today and comparing it to HN MtDNA might be irrelevant because a vast amount of presumable variation was lost when our species almost became extinct (and current variation is the result of drift due to the small bottleneck population).

I could be wrong about the number, but the homogeneity of our species' DNA is something I'm almost sure of (Dawkins mentioned it in "The Ancestor's Tale", for example) - and this is definitely relevant to HN since HN and HS co-existed, probably, for tens of thousands, if not more, years.

True, I also recall having read that much more recently than the Neanderthal-human split the human population was as low as 2,000 individuals. So perhaps we should therefore expect the human mtDNA variation to be less than it was at the time of the split. In other words, you're not convinced that interbreeding can be ruled out yet, and I guess I would have to agree.

Unless I am missing some point, which I discover about myself quite often, so that's totally possible.

They have currently only partially ruled out interbreeding in humans based on mtDNA. Now they are investigating nuclear DNA to see if any signal of admixture exists there. This is one of the primary questions the Neanderthal genome people (Svante Paabo and others) are trying to answer. But if Humans are homogeneous and have no markers related to Neanderthals (in the ntDNA, Paabo gave a preliminary conclusion of the work at SMBE meeting last summer) and it was lost during the bottleneck than interbreeding doesn't matter anymore for human evolution because Neanderthal's decedents were the ones lost during the bottle neck. Also, it means that there can never be a conclusive result of admixture because we would have no signal of it.

If there was admixture and the bottleneck purged it out of our ancestral population we will never be able to know admixtures impact, if any, on the fate of Neanderthals.

Pleiotropy comes from the Greek πλείων pleion, meaning "more", and τρέπειν trepein, meaning "to turn, to convert". It designates the occurrence of a single gene affecting multiple traits, and is a hugely important concept in evolutionary biology.

I'm a postdoc at UC Santa Barbara.

All Many aspects of evolution interest me, but my research focus is currently on microbial evolution, adaptive radiation, speciation, fitness landscapes, epistasis, and the influence of genetic architecture on adaptation and speciation.